Edward N. Peters, an authority on canon law, states unequivocally on his blog that On-line confessions are absolutely null and utterly void. Even if there's a real priest on the line, he writes, “the confessions themselves are of absolutely zero sacramental value.”
Not to mention that you don't know who's on the other end of the line — and if it's someone pretending to be a priest then this could be the best blackmail wheeze since the cleartext-only anonymous remailer that secretly kept logs.
Why are modem-mediated confessions worthless? Apparently a key reason is Canons 960 & 961, although how they prove it is … I confess … lost on me. (I would get it if the reason were Canon 964.)
Let me, by the way, say that to this non-Catholic, Edward N. Peters's blog In the Light of the Law: a canon lawyer's blog on current issues makes very interesting reading. Often I'm following right along, whether or not I agree with the assumptions — lots of legal analysis has similar properties — and then every so often there's stuff that shows such a very different mindset and instincts from mine at work…
Incidentally, I don't in fact know of an cleartext-only anonymous remailer that secretly kept logs, but I always thought it would make a fine idea for a novel if the money-making blackmail thing didn't work out.